LTE-Advanced (Long Term Evolution Advanced) wireless communications systems make use of carrier aggregation (CA) to increase bandwidth which in turn results in increased bitrates. Interband downlink (DL) CA requires radio reception in two or more radio receive (Rx) bands simultaneously while transmitting in one or the other of the two bands. These two or more Rx bands are at different frequencies. Depending on the frequency separation and configuration of the Rx bands, an antenna or antennas must be tuned with an RF (radio frequency) impedance matching network to resonate at the required frequencies.
In a mobile device, the maximum usable antenna bandwidth is determined by design constraints such as the physical size and structure of the antenna and the matching networks. In one known implementation, a single antenna is designed with sufficient bandwidth to accommodate both frequency bands including transmit (Tx) and Rx sub-bands at the same time with reasonable performance. In such single antenna implementations, the full desired bandwidth of the antenna cannot be met due to the limited size of the physical antenna. In addition, duplex filters for both bands must be combined through diplexing which has its own drawbacks in terms of additional insertion loss and bandwidth.
An example of such a known single antenna system 100 is shown in FIG. 1. A single antenna 101 that is designed for wide bandwidth is coupled to an antenna matching network 103 on the antenna feed. The antenna matching network 103 is connected to a band switch 107 through an RF test connector 105. The band switch 107 is operative to switch the antenna 101 to various band feeds to a transceiver 160. The example includes a band A & B feed 109, band X feed 123, and band Y feed 125, and a band Z feed 117. For LTE-Advanced operation, the band A & B feed 109 is used and one of the bands A or B may be used as a Primary Component Carrier (PCC) with the other of the bands being used as a Secondary Component Carrier (SCC).
The band A & B feed 109 is coupled to a quadplexer 111, which is high insertion loss diplexed duplex filters, for the two bands. The quadplexer 111 is coupled to a power amplifier (PA) 113 and low noise amplifier (LNA) 114 for the corresponding respective Tx and Rx connections for band A, and to a PA 115 and LNA 116 for the corresponding respective Tx and Rx connections for band B.
For operation without using CA, the band switch 107 may be switched to the band Z feed 117. The band Z feed 117 is coupled to a duplexer 119. The duplexer 119 is coupled to a PA 121 and an LNA 122 for the corresponding respective Tx and Rx connections for band Z. The band switch 107 may also switch to the band X feed 123, or to the band Y feed 125 for reception on those bands.
The primary drawback of the single antenna system 100, is that the antenna 101 cannot provide the full desired bandwidth to accommodate operation in all five bands, A, B, X, Y and Z, because of design constraints on antenna size. The antenna matching network 103 is also subject to design constraints and can only provide a certain amount of antenna operational improvement. Another drawback is the requirement of the quadplexer 111 to support PCC and SCC operation, because the quadplexer 111 has a high insertion loss.
In another known implementation, a second antenna can be dedicated to the second band all the time, which eliminates the need for the high insertion loss quadplexer and allows for antenna designs that are better matched to the desired bandwidths. An example of such a known two antenna system 200 is shown in FIG. 2. The example includes a band A feed 209, band X feed 215, band B feed 229, band Y feed 241, and a band Z feed 235.
A moderate bandwidth first antenna 201 has a corresponding antenna matching network 203 on the antenna feed and is operatively coupled to a band switch 207 through an RF test connector 205. The band switch 207 is operative to switch the first antenna 201 to band A feed 209 and band X feed 215. A moderate bandwidth second antenna 221 has a corresponding antenna matching network 223 on the antenna feed and is operatively coupled to a band switch 227 through an RF test connector 225. The band switch 227 is operative to switch the second antenna 221 to band B feed 229 and band Z feed 235. For LTE-Advanced operation, one of band A or band B is used as a PCC with the other of the bands being used as an SCC. In the two antenna system 200, the first antenna 201 is designed to cover the full transmit and receive bandwidths for band A and band X, while the second antenna 221 is designed to cover the full transmit and receive bandwidths for band B, band Y and band Z.
The band A feed 209 is coupled to a first duplexer 211, which is in turn coupled to a PA 213 and an LNA 214 for the corresponding respective Tx and Rx connections for band A. The band X feed 215 is coupled to a second duplexer 217, which is in turn coupled to a PA 219 and an LNA 220 for the corresponding respective Tx and Rx connections for band X. For the second antenna 221 transceiver front end, the band B feed 229 is coupled to a third duplexer 231, which is in turn coupled to a PA 233 and an LNA 234 for the corresponding respective Tx and Rx connections for band B. The band Z feed 235 is coupled to a fourth duplexer 237, which is in turn coupled to a PA 239 and an LNA 240 for the corresponding respective Tx and Rx connections for band Z. All of the PAs and LNAs are operatively coupled to a transceiver 260.
For operation without CA, one of the band switches must be switched to one of the desired bands A, X, B, Y, or Z for example where one antenna is used to receive and transmit in the desired band, but the other, second antenna, while physically present, is not being utilized at that instant. The second antenna, although not being used at that instant, is designed to support the bands assigned to that antenna and may have a natural response for all the bands it is designed to support. If the bands on the first antenna are close in frequency to the second antenna, there will be degraded antenna efficiency due to finite antenna coupling. This antenna coupling and efficiency degradation is made worse if the antenna switch on the second antenna is connected to the duplex filter that is close in frequency to the active band. Another drawback of the two antenna system 200 is that the antenna matching networks may still be complex. If band A and band B are relatively close to each other and band X, Y and Z are close to each other, then the antenna matching networks are similar and doubled compared to the single antenna system 100. It is desirable to reduce the complexity of one of the antenna matching networks, either antenna matching network 203 or antenna matching network 223.